To reduce the weight of automobile bodies, high-strength steel sheets are actively used as materials of automobile components. Steel sheets used for structural members and reinforcing members of automobiles are mainly steel sheets having a tensile strength (TS) of 780 MPa or more. To strengthen a steel sheet, it is effective to utilize structural strengthening, and one method is to form a complex structure including soft ferrite and hard martensite. Generally, a steel sheet having this complex structure has good ductility, an excellent strength-ductility balance (compatibility), and relatively good press formability. However, in the steel sheet having the complex structure, its material properties such as tensile strength (TS) vary greatly in response to changes in the conditions of production such as annealing temperature in an ordinary continuous annealing line, and therefore the material properties tend to vary in the lengthwise direction of a coil.
The high-strength steel sheets used as materials of automobile components are subjected to punching and therefore required to have punchability. The required punchability is that the amount of wear of the punch and die is small, i.e., continuous punchability is excellent, and that variations in press formability of punched edge faces and their vicinities when continuous punching is performed are small.
When punching is performed in an actual component production process, it is difficult to control clearance (the clearance between the punch and die) to be constant because of issues such as the mounting accuracy of the die, and the clearance fluctuates within the range of 5 and 20%. Therefore, when variations in material properties within a single coil or between coils are large, variations in press formability of punched edge faces formed by punching and their vicinities are large. In this case, it is difficult to perform press forming stably in a continuous press line for automobiles. When stable press forming is difficult, it is feared that workability may deteriorate significantly.
In a highly formable high-strength cold-rolled steel sheet described in Patent Literature 1, certain amounts of Si and Al are added to reduce changes in structure caused by fluctuations in annealing conditions to thereby reduce variations in mechanical properties such as elongation and stretch flangeability.
Patent Literature 2 discloses a hot-rolled steel sheet in which deterioration in punchability is prevented by reducing the contents of Ti-based nitrides in the steel sheet.